Generally, milling tools and the milling inserts thereof are used for chip removing or cutting machining of workpieces of various metallic materials, such as steel, aluminum, titanium, etc., which in practice have most varying properties in respect of, for instance, workability, generation of heat, sticking, and so on. Milling inserts of the kind initially mentioned (known as “router inserts” by those skilled in the art) are especially suitable for milling in easily machined materials, such as aluminum, in which the milling inserts have great cutting capacity and generate the smoothest possible surfaces. For this reason, the milling inserts are manufactured having a positive cutting geometry so far that the cutting edge angle between the individual chip surface and the appurtenant clearance surface is acute (e.g., within the range of 65-75°), at the same time as the milling inserts are mounted with positive tipping-in angles in the appurtenant seatings in the milling cutter body as viewed axially as well as radially. In such a way, the milling inserts can “dig themselves” into the material in an efficient and powerful way, and thereby remove large amounts of chips per time unit. In order to generate smooth (usually plane, curved or cylindrical) surfaces, the main edge of the individual cutting edge has to be given a convexly curved or cambered shape as viewed in plane elevation, so that the cutting edge line of the main edge, in spite of the axially positive tipping-in of the milling inserts into the milling cutter body will follow or be tangent to an imaginary cylinder, the diameter of which is determined by the radial distance between the rotation axis of the milling cutter body and the cutting edge line of the main edge. It should also be mentioned that the milling inserts in question, by having at least two serviceable cutting edges, and thereby being indexable, get a service life that is at least twice as large as those milling inserts only having one cutting edge.
A problem with previously known, high-capacity milling inserts (see, for instance, WO/02/055245 A1 and SE 527617 C2) is, however, that the same under certain circumstances may be subjected to vibrations, which, if they become regenerative and are allowed to grow in strength in an uncontrolled way, may lead to drastic consequences, such as tool or machine breakdowns, rejections of expensive workpieces, personal risks of injury, etc. The risk of emergence of vibrations is particularly great when the milling tool is slender and operates using great cutting depths, e.g., in aluminum. As long as the cutting depths are small or moderate (up to 2 mm), initial vibration tendencies usually die out by themselves, but when the cutting depths increase, for instance to ¼ of the length of the cutting edge or more, from which it follows that also the cutting forces are enhanced, it often occurs that the vibrations are fed back and grow in an uncontrolled way. The risk of vibration is particularly marked for tools of steel, such as shank-end mills, the length of which is many times greater than the diameter (L>3×D).
The present invention aims at overcoming the above-mentioned problems and at providing an improved milling insert of the indexable type. An object of the invention to provide a milling insert manufactured from cemented carbide or another hard and wear-resistant material, which has an inherent ability to automatically damp or deaden vibrations as soon as these tend to arise. The ultimate purpose of counteracting the emergence of vibrations is to make it possible to increase the axial cutting depth and thereby the removal volume, without risking damage the tool and/or the machine by escalating vibrations.
Another object of the invention is to realize the vibration damping function by ways that are technically simple and thereby inexpensive.
Yet another object of the invention is to provide a milling insert that is particularly suitable to use in shank-end mills.